Recent results in the gerbil have demonstrated dramatic reductions of hemispheric blood flow, assessed by a (3H) nicotine tracer diffusion method, following placement of a single electrode in striatum for the purpose of measuring blood flow by the alternative hydrogen clearance method. Blood flow in cortical regions of the impaled hemisphere is reduced several-fold relative to the opposite hemisphere, returning gradually to control levels by 3-6 hr. Blood flow measurements in striatum, obtained with the implanted hydrogen clearance electrode, also increases two- fold during 6 hr after placement. These results demonstrate that significant reductions of normal blood flow arise following acute placement of electrodes for hydrogen clearance measurements in the gerbil, although chronic implantation may still provide valid measurements. Measurements obtained in cats several years ago with chronically implanted hydrogen electrodes gave unexpectedly high values for striatal blood flow, attributed at that time to the light anesthesia which could be used at the time of measurement. These results will be systematically reinvestigated in view of the current findings in the gerbil.